This invention is concerned with cable suspended pumping systems and is more particularly concerned with the provision of safety apparatus in such systems.
In tubing suspended submergible pumping systems for oil wells it is conventional to provide a pack-off tubing hanger assembly that includes a wireline retrievable surface controlled safety valve, run 100 feet below the mud line, for example, to keep the well under control in the event of damage to the wellhead. A control line may be used to set the hanger in the well and to actuate the safety valve. In addition to the production fluid path controlled by the safety valve, the hanger-valve installation may include an annulus gas vent, a cable by-pass, and a fluid by-pass that allows opening and closing of the safety valve without shutting off the electric pump suspended from the pack-off tubing hanger.
The advantages of this type of construction have not heretofore been available in cable suspended pumping systems, i.e., systems in which the submergible pumping apparatus is suspended by a cable, rather than by tubing. An advantage of cable suspended pumping systems is that they can be run into or out of a well by cable winches, eliminating the need for long lengths of tubing in deep wells and eliminating the need for assembling and disassembling tubing sections during installation and removal of the pumping apparatus.
In the past, safety valves in cable suspended pumping systems have been part of the downhole pumping unit, usually thousands of feet beneath the earth's surface. Because of the inaccessibility of the safety valves in such systems, the valves are operated in response to pump discharge pressure. This requires a non-standard design and one that will not operate in all well conditions. To operate the valves properly in response to pump discharge pressure it is necessary that the pump build up pressure rapidly. This can be achieved when the well fluid is rather incompressible, but most wells contain significant amounts of natural gas dissolved in oil, or as free gas in the well. Under these circumstances the column of fluid above the pump is very compressible, and pump pressure builds up very slowly.